Power supply apparatus for e.g. a plasma arc cutter or an arc welder, includes input terminals between which an AC voltage is applied. The AC voltage is rectified by an input-side rectifier and smoothed by a smoothing capacitor, and then converted into a DC voltage. The DC voltage is converted into a high frequency voltage by an inverter. The high frequency voltage is, then, voltage-transformed by a transformer. The transformed high frequency voltage is rectified by an output-side rectifier and, then, applied to a load. In such power supply apparatus, an inverter is used for converting a DC voltage into a high frequency voltage, and, therefore, a small transformer can be used, whereby the power supply apparatus can be downsized as a whole.
When such power supply apparatus is used for e.g. a plasma arc cutter, it has a power factor of e.g. 0.5-0.6 when operated from rated voltage and rated current. The power supply apparatus includes a breaker or fuse coupled to the input terminals thereof for circuit protection. The fuse has an interrupting capacity of e.g. 15 A. Thus, the power supply apparatus allows a maximum of 15 A of current to flow thereinto. Assuming that an input AC voltage is 115 V, an input current is 15 A, the plasma arc cutter has an efficiency of 85%, and the power factor is 0.6, the maximum possible output power P is 880 W. The output voltage of the plasma arc cutter should be e.g. 120 V due to the characteristics of a load coupled thereto. Then, the output current available is about 7.3 A. Thus, the power supply apparatus cannot provide large output current for the input current applied thereto.
Japanese Patent Application No. HEI 8-327642 filed on Nov. 22, 1996, which was published under Japanese Unexamined Patent Publication (KOKAI) No. HEI 10-156537 on Jun. 16, 1998 and corresponds to U.S. patent application Ser. No. 08/968,054, discloses a power supply apparatus, which may solve the above-described problem. In that power supply apparatus, an AC signal is converted into a DC signal by an AC-to-DC converter. The DC signal has its power factor improved by a high power-factor converter. The DC signal with the improved power factor is converted into a high frequency signal by an inverter. The high frequency signal is converted into a DC signal by a high-frequency-to-DC converter and applied to a load. An output current of the high-frequency-to-DC converter is detected by a current detector, and also output power of the high-frequency-to-DC converter is detected by a power detector. When the power supplied to the load is lower than a first preset value, a control unit receives a signal representative of the output current detected by the current detector and controls the inverter such that the output current representative signal becomes equal to a second preset value. When the power supplied to the load is higher than the first preset value, the control unit receives a signal representative of the output power detected by the power detector and controls the inverter such that the output power representative signal becomes equal to the second preset value.
In this power supply apparatus, when there is a large gap between a torch and a workpiece constituting the load, for example, and current flows through the gap, the output voltage and the output current become higher. This causes power higher than the first preset value to be supplied to the torch and the workpiece. Then, the output power representative signal, which represents the higher power, is supplied to the control unit, which controls the inverter in such a manner that the output power becomes constant. Under such constant-power control, as the gap is made smaller, the output current increases and the output voltage decreases. When the power supplied to the torch and the workpiece decreases below the first preset value, the output current representative signal, which represents the increased current, is coupled to the control unit, which, then, controls the inverter in such a manner that the output current is kept constant.
In the power supply apparatus disclosed in Japanese Patent Application No. HEI 8-327642, the control of the inverter is switched between the constant-power control and the constant-current control, and, therefore, the output voltage when the control of the inverter is switched from the constant-power control to the constant-current control can be maintained to be low. As a result, a relatively large current can be supplied to the load under the constant-current control, so that the power supplied to the power supply apparatus can be efficiently used. In addition, the use of the high power-factor converter can improve the power factors of the current and the voltage to be supplied to the inverter to thereby increase the power to be supplied to the load.
In the above-described power supply apparatus, however, the constant-current control is provided when the power supply apparatus operates at the rated voltage and current. Therefore, the power supply apparatus tends to provide a constant output current even when the input voltage is varied. For example, the plasma arc cutter providing an output voltage of 80 V and an output current of 15 A when the input voltage is 100 V and the input current is 15 A tends to keep the output current at 15 A even when the input voltage is varied to 90 V, so that the input current increases to about 16.6 A. When the input voltage is varied to 85 V, the input current increases to 17.6 A. Therefore, the breaker or fuse having an interrupting capacity of 15 A, which is coupled to input terminals of the power supply apparatus, is tripped.
An object of the present invention is to provide power supply apparatus which can keep an input current constant even if an input voltage varies.